Relativistic Electrodynamics

Continuity Equation

Recall the continuity equation J=ρt. In terms of the four-current density, the left hand side is J=Jxx+Jyy+Jzz=3i=1Jixi, while the right hand side is ρt=1cJ0t=J0x0. Thus, the continuity equation in terms of the four-current density is Jμxμ=0. As this is also the four-dimensional divergence of Jμ, the continuity equation implies the four-current density is divergenceless.

Maxwell's Equations in Terms of Field Tensor

Recall the Maxwell's equations E=ρϵ×B=μ0J+μ0ϵ0EtB=0×E=Bt. For equation (1), let us examine F0νxν F0νxν=F00x0+F01x1+F02x2+F03x3=1c(Exx0+Eyy+Ezz)=1c(E)=μ0cρ=μ0J0. Thus, F0νxν=μ0J0 is equivalent to equation (1).

For equation (2), let us examine F1νxν F1νxν=F10x0+F11x1+F12x2+F13x3=1c2Ext+BzyByz=(1c2Et+×B)x=μ0Jx=μ0J1. Similarly for the F2νxν and F3νxν. Thus, Fiνxν=μ0Ji is equivalent to the equation (2).

For equation (3), let us examine G0νxν G0νxν=G00x0+G01x1+G02x2+G03x3=Bxx+Byy+Bzz=B=0. Thus, G0νxν=0 is equivalent to equation (3).

For equation (4), let us examine G1νxν G1νxν=G10x0+G11x1+G12x2+G13x3=1cBxt1cEzy+1cEyz=1c(Bt+×E)x=0. Similarly for the G2νxν and G3νxν. Thus, Giνxν=0 is equivalent to equation(4).

Therefore, the Maxwell's equations in terms of the four-current density are {Fμνxν=μ0JμGμνxν=0

Maxwell's Equations in terms of 4-vector Potential

Substitute Fμν=AνxμAμxν into Fμνxν=μ0Jμ, we have xμ(Aνxν)xν(Aμxν)=μ0Jμ. As Fμν is gauge invariant (as E and B are gauge invariant), the gauge transformation A^\mu\to A^{\mu}'=A^\mu + \frac{\partial \lambda}{\partial x_\mu} leaves Fμν unchanged. The Lorentz gauge condiiton A=1c2Vt in terms of 4-vector potential is Aνxν=0. Therefore, (5) reduces to 2Aμ=μ0Jμ, where is the d'Alembertian defined as 2:=xνxν=21c22t2p

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